Class 10 Science 2025 Board Exam

Electrolysis of water is a decomposition reaction. The mass ratio
($M_H:M_O$) of hydrogen and oxygen gases liberated at the electrodes
during electrolysis of water is :

Assertion (A) : Decomposition reactions are generally endothermic reactions.
Reason (R) : Decomposition of organic matter into compost is an exothermic process.

A student performs the following experiment in his school laboratory.
List two observations to justify that in this experiment a chemical change has taken place.

What happens when : (write balanced chemical equation)
(a) Lead nitrate is thermally decomposed,
(b) Natural gas burns in oxygen (or air) ?

Translate the following statements into chemical equations and then balance them :
(a) Nitric acid reacts with calcium hydroxide to form calcium nitrate and water.
(b) Sodium chloride reacts with silver nitrate to form silver chloride and sodium nitrate.

The correct balanced chemical equation showing exothermic reaction in which natural gas burns in air is :

Balance the following chemical equations :
(a) $\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\text{Sunlight/Chlorophyll}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$
(b) $\mathrm{Pb}(\mathrm{N}{\mathrm{O}}_3{)}_2\stackrel{\text{Heat}}{\to }\mathrm{PbO}+\mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$

(a) (i) Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity.
(ii) Decomposition of vegetable matter into compost is considered an exothermic reaction. Why ?
OR
(b) Why are decomposition reactions called the opposite of combination reactions ? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.

In the electrolysis of water, the mass ratio of hydrogen and oxygen gases liberated at the electrodes is :

The following table shows the pH values of four solutions A, B, C and D on a pH scale :
1 2 3 4 5 6 7 8 9 10 11 12 13 14
The solutions A, B, C and D respectively are of a